Hydrogen sulfide

Hydrogen sulfide is a covalent hydride structurally related to water (H2O) since oxygen and sulfur occur in the same periodic table group.

Hydrogen sulfide is weakly acidic, dissociating in aqueous solution into hydrogen cations H+ and the hydrosulfide anion HS−:

H2S → HS− + H+

Ka = 1.3×10−7 mol/L; pKa = 6.89.

The sulfide ion, S2−, is known in the solid state but not in aqueous solution (c.f. oxide). The second dissociation constant of hydrogen sulfide is often stated to be around 10−13, but it is now clear that this is an error caused by oxidation of the sulfur in alkaline solution. The current best estimate for pKa2 is 19±2.[1]

Hydrogen sulfide reacts with many metals cations to produce the corresponding metal sulfides. Well-known examples are silver sulfide (Ag2S), the tarnish that forms on silver when exposed to the hydrogen sulfide of the atmosphere, and cadmium sulfide (CdS), a pigment also known as cadmium yellow. Transition metal sulfides are characteristically insoluble, thus H2S is commonly used to separate metal ions from aqueous solutions. (Sulfides should not be confused with sulfites or sulfates, which contain the sulfite ion SO32− and the sulfate ion SO42−, respectively.)

Hydrogen sulfide is corrosive and renders some steels brittle, leading to sulphide stress cracking — a concern especially for handling "sour gas" and sour crude oil in the oil industry. Hydrogen sulfide burns to give the gas sulfur dioxide, which is more familiar as the odor of a burnt match.
Small amounts of hydrogen sulfide occur in crude petroleum but natural gas can contain up to 90%. Volcanoes and hot springs emit some H2S, where it probably arises via the hydrolysis of sulfide minerals, i.e. MS + H2O to give MO + H2S. Normal concentration in clean air is about 0.0001-0.0002 ppm.[citation needed]

Sulfate-reducing bacteria obtain energy by oxidizing organic matter or hydrogen with sulfates, producing H2S. These microorganisms are prevalent in low-oxygen environments, such as in swamps and standing waters. Sulfur-reducing bacteria (such as Salmonella) and some archaea obtain their energy by oxidizing organic matter or hydrogen with elemental sulfur, also producing H2S. Other anaerobic bacteria liberate hydrogen sulfide when they digest sulfur-containing amino acids, for instance during the decay of organic matter. H2S-producing bacteria also operate in the human colon, and the odor of flatulence is largely due to trace amounts of the gas. Such bacterial action in the mouth may contribute to bad breath. Evidence exists that hydrogen sulfide produced by sulfate-reducing bacteria in the colon may cause or contribute to ulcerative colitis.

About 10% of total global emissions of H2S are due to human activity. By far the largest industrial route to H2S occurs in petroleum refineries: the hydrodesulfurization process liberates sulfur from petroleum by the action of hydrogen. The resulting H2S is converted to elemental sulfur by partial combustion via the Claus process, which is a major source of elemental sulfur. Other anthropogenic sources of hydrogen sulfide include coke ovens, paper mills (using the sulphate method), and tanneries. H2S arises from virtually anywhere where elemental sulfur comes into contact with organic material, especially at high temperatures.

Hydrogen sulfide can be present naturally in well water. In such cases, ozone is often used for its removal. An alternative method uses a filter with manganese dioxide. Both methods oxidize sulfides to less toxic sulfates.